Optical recording medium and method of recording information to optical recording medium

ABSTRACT

It is an object of the present invention to provide an optical recording medium that is suitable for recording information therein at a high velocity. An optical recording medium according to the present invention includes a recording layer formed of a phase change material and capable of recording data therein at a linear recording velocity equal to or higher than 10 m/sec, wherein Rtop satisfies the condition that it is larger than {11−(V/5)} and smaller than {22−(2V/5)}, where Rtop (%) is a reflectivity of the crystal phase change material forming the recording layer and V (m/sec) is a target linear recording velocity. According to the present invention, since modulation (MOD) of 50% or more can be ensured, it is possible to effectively suppress the degradation of jitter caused by recording data at a high linear recording velocity equal to or higher than 10 m/sec.

BACKGROUND OF THE INVENTION

The present invention relates to an optical recording medium andparticularly to an optical recording medium that is suited to recordinformation therein at a high velocity. In addition, the presentinvention relates to a method of recording information to an opticalrecording medium that can record information therein at high datatransfer rates.

DESCRIPTION OF THE PRIOR ART

Optical recording media typified by the CD and the DVD and the like havebeen widely used as recording media for recording digital data, and awidely used data recording format is a format wherein the lengths ofrecording marks along the track are modulated depending on the data tobe recorded. For example, in a DVD-RW which is one type of opticalrecording medium whose data is user-rewritable, recording marks oflengths corresponding to 3T to 11T (where T is one clock cycle) are usedto perform the recording of data.

When a recording mark is formed, a laser beam is shined along the tracksof the optical recording medium, thereby forming an amorphous regionhaving a predetermined length in a recording layer included in anoptical recording medium and the thus formed amorphous region isutilized as a recording mark. Other regions of the recording layer thanthe amorphous region are in crystal phase.

When a recording mark is to be formed in a recording layer, a laser beamwhose power is set to a high level (a recording power) is projected ontothe recording layer, thereby heating the recording layer to atemperature higher than the melting point thereof and the recordinglayer is then quickly cooled. As a result, the phase of the recordinglayer is changed from crystal phase to amorphous phase and a recordingmark is formed in the recording layer. On the other hand, when arecording mark is to be erased, a laser beam whose power is set to arelatively low level (an erasing power) is projected onto the recordinglayer, thereby heating the recording layer to a temperature equal to orhigher than the crystallization temperature thereof and the recordinglayer is then gradually cooled. As a result, the phase of the recordinglayer is changed from the amorphous phase to the crystal phase and therecording mark is erased. Therefore, if the power of the laser beam ismodulated in this manner, it is possible to not only form a recordingmark in an unrecorded region of the recording layer but also directlyoverwrite (direct-overwrite) a recording mark already formed in a regionof the recording layer with a different recording mark.

In recent years, it has become highly desirable to achieve furtherincreases in the data transfer rate with respect to optical recordingmedia and in order to achieve this, it is effective to form a recordinglayer of a phase change material having a high crystallization velocity.

However, in a phase change material having a high crystallizationvelocity, the difference between the absolute reflectivity thereof inthe crystal phase and that in the amorphous phase is generally small andwhen recorded data are reproduced, good jitter characteristics of thereproduced signal cannot be obtained. In addition, since the width of arecording mark becomes smaller as the linear recording velocity isincreased in order to increase the data transfer rate, the degradationof jitter caused by decrease in difference in absolute reflectioncoefficient becomes pronounced as the linear recording velocityincreases and the degradation of jitter becomes particularly seriouswhen the linear recording velocity is equal to or higher than 10 m/sec.

Therefore, although the degradation of jitter caused by decrease indifference in absolute reflection coefficient is not so serious when thelinear recording velocity is lower than 10 m/sec, the jittercharacteristics drastically deteriorate due to decrease in difference inabsolute reflection coefficient when the linear recording velocity isequal to or higher than 10 m/sec.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide anoptical recording medium that is suitable for recording informationtherein at a high velocity.

In addition, it is another object of the present invention to provide amethod of recording information to an optical recording medium that canrecord information in the optical recording medium at high data transferrates.

The difference in reflection coefficient between that of a recordinglayer formed of a phase change material in the crystal phase and that ofthe recording layer formed of the phase change material in the amorphousphase is generally expressed by a parameter called modulation (MOD) anda greater output amplitude can be obtained when information isreproduced from a recording layer having a higher modulation (MOD).Here, the modulation (MOD) is defined as follows.MOD=(Rtop−Rbtm)/Rtop  (1)where Rtop is the reflectivity of the phase change material in thecrystal phase and Rbtm is the reflectivity of the phase change materialin the amorphous phase. Here, Rtop and Rbtm are respectively defined asthe reflectivity of a crystalline region and the reflectivity of anamorphous region when tracking of a condensed laser beam is performedalong a groove.

As apparent from the equation (1), the modulation (MOD) can be increasedby increasing the absolute difference between Rtop and Rbtm ordecreasing Rtop. However, as described above, since the absolutedifference between Rtop and Rbtm becomes small as the crystallizingvelocity of a phase change material increases, it is necessary todecrease Rtop in order to increase the modulation (MOD). This can beachieved by adjusting the thickness of a dielectric layer(s), forexample.

Based on this technical teaching, the inventors of the present inventionfound a preferable relationship between a target (attainable) linearrecording velocity and the reflectivity of a recording layer in thecrystal phase and completed the present invention.

Specifically, the above mentioned-object of the present invention can beaccomplished by an optical recording medium comprising a recording layerformed of a phase change material and capable of recording data thereinat a linear recording velocity equal to or higher than 10 m/sec, whereinRtop satisfies the condition that it is larger than {11−(V/5)} andsmaller than {22−(2V/5)}, where Rtop (%) is a reflectivity of thecrystal state of the phase change material forming the recording layerand V (m/sec) is a target linear recording velocity.

Further, the above mentioned object of the present invention can beaccomplished by an optical recording medium comprising a recording layerformed of a phase change material and capable of recording data thereinat a linear recording velocity equal to or higher than 10 m/sec andhaving recording condition setting information related to a linearrecording velocity V (m/sec) to be set when recording data, wherein Rtopsatisfies the condition that it is larger than {11−(V/5)} and smallerthan {22−(2V/5)}, where Rtop (%) is a reflectivity of the crystal stateof the phase change material forming the recording layer.

According to the present invention, since modulation (MOD) of 50% ormore can be ensured, it is possible to effectively suppress thedegradation of jitter caused by recording data at a high linearrecording velocity equal to or higher than 10 m/sec.

In a preferred aspect of the present invention, Rtop and V satisfy thecondition that Rtop is larger than {11−(V/5)} and smaller than{20−(4V/11)}.

According to this preferred aspect of the present invention, sincemodulation (MOD) of 55% or more can be ensured, it is possible to moreeffectively suppress the degradation of jitter caused by recording dataat a high linear recording velocity equal to or higher than 10 m/sec.

In a further preferred aspect of the present invention, Rtop and Vsatisfy a condition that Rtop is larger than {11−(V/5)} and smaller than{18.3−(V/3)}.

According to this preferred aspect of the present invention, sincemodulation (MOD) of 60% or more can be ensured, it is possible to moreeffectively suppress the degradation of jitter caused by recording dataat a high linear recording velocity equal to or higher than 10 m/sec.

The above object of the present invention can be also accomplished by amethod of recording information to an optical recording mediumcomprising a recording layer formed of a phase change material whosereflectivity is Rtop (%) when it is in a crystal phase, which methodrecords information under the condition that Rtop is larger than{11−(V/5)} and smaller than {22−(2V/5)}, where V (m/sec) is a linearrecording velocity.

According to the present invention, since modulation (MOD) of 50% ormore can be ensured, it is possible to effectively suppress thedegradation of jitter caused by recording data at a high linearrecording velocity equal to or higher than 10 m/sec.

In a preferred aspect of the present invention, information is recordedunder the condition that Rtop is larger than {11−(V/5)} and smaller than{20−(4V/11)}.

According to this preferred aspect of the present invention, sincemodulation (MOD) of 55% or more can be ensured, it is possible to moreeffectively suppress the degradation of jitter caused by recording dataat a high linear recording velocity equal to or higher than 10 m/sec.

In a further preferred aspect of the present invention, information isrecorded under the condition that Rtop is larger than {11−(V/5)} andsmaller than {18.3−(V/3)}.

According to this preferred aspect of the present invention, sincemodulation (MOD) of 60% or more can be ensured, it is possible to moreeffectively suppress the degradation of jitter caused by recording dataat a high linear recording velocity equal to or higher than 10 m/sec.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the relationship between linear recordingvelocities attainable by phase change materials and absolute difference(Rtop−Rbtm) between Rtop and Rbtm thereof.

FIG. 2 is a graph showing the relationship between linear recordingvelocities attainable when modulation (MOD) is 50%, 55% or 60%, and thereflectivity (Rtop) of a crystal phase change material.

FIG. 3 is a drawing schematically showing the structure of an opticalrecording medium 10 that is a preferred embodiment of the presentinvention.

FIG. 4 is a drawing schematically showing the major components of adrive.

FIG. 5 is a drawing illustrating the pulse train pattern in the case offorming a recording mark of a length corresponding to 2T.

FIG. 6 is a graph showing the relationship between the reflectivity(Rtop) of a crystal phase change material and jitter.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A preferred embodiment of the present invention will be explained indetail with reference to the drawings.

As pointed out above, it is necessary to increase the crystallizingvelocity of a phase change material forming a recording layer in orderto record data in an optical recording medium at a high data transferrate and it is possible to see a substantially constant relationshipbetween linear recording velocities attainable by phase change materialsand absolute difference between Rtop and Rbtrn thereof.

FIG. 1 is a graph showing the relationship between linear recordingvelocities attainable by phase change materials and absolute difference(Rtop−Rbtm) between Rtop and Rbtm thereof.

It can be seen from FIG. 1 that the absolute difference between Rtop andRbtm of a phase change material decreases as the attainable linearrecording velocity of the phase change material increases and that thistendency is substantially linear. This linear relationship can beexpressed by the following equation (2):Rtop−Rbtm=11−(V/5)  (2)

On the other hand, as pointed out above, in order to obtain aconsiderably high output amplitude when reproducing data, it iseffective to increase the modulation (MOD) of the recording layer. Inparticular, since the width of a recording mark becomes smaller as thelinear recording velocity increases, it is necessary to considerablyincrease the modulation (MOD) of the recording layer in the case wherethe linear recording velocity is equal to or higher than 10 m/sec.Concretely, in the case where the linear recording velocity is equal toor higher than 10 m/sec, modulation (MOD) of 50% or more is required inorder to obtain considerably high output amplitude and the modulation(MOD) is preferably equal to or higher than 55%, more preferably equalto or higher than 60%.

The conditions required for achieving modulation (MOD) of 50% or morecan be derived from formulae (1) and (2). Specifically, modulation (MOD)of 50% or more can be ensured if the reflectivity (Rtop) of the crystalphase change material satisfies the following formula (3).Rtop<22−(2V/5)  (3)

Further, since it follows from formula (1) that the lower limit of Rtopis obtained when the reflectivity (Rbtm) of the amorphous phase changematerial becomes zero, Rtop can be expressed by the following formula(4).Rtop>11−(V/5)  (4)

Moreover, the conditions required for achieving modulation (MOD) of 55%or more and those required for achieving modulation (MOD) of 60% or morecan be derived from formulae (1) and (2). Specifically, modulation (MOD)of 55% or more can be ensured if the reflectivity (Rtop) of the crystalphase change material satisfies the following formula (5) and modulation(MOD) of 60% or more can be ensured if the reflectivity (Rtop) of thecrystal phase change material satisfies the following formula (6).Rtop<20−(4V/11)  (5)Rtop<18.3−(V/3)  (6)

FIG. 2 is a graph corresponding to the above formulae (3) to (6) showingthe relationship between linear recording velocities attainable when themodulation (MOD) is 50%, 55% or 60%, and the reflectivity (Rtop) of acrystal phase change material.

As apparent from FIG. 2, the value to be set as the reflectivity (Rtop)of a crystal phase change material decreases as the attainable linearrecording velocity increases. Concretely, in the case where a datatransfer rate of 70 Mbps taking the format efficiency to beapproximately 80% is achieved using the (1,7) RLL modulation code, sincethe required linear recording velocity is 10.5 m/sec, the value of Rtoprequired for achieving modulation (MOD) of 50% or more is 8.7% to 14.7%,the value of Rtop required for achieving modulation (MOD) of 55% or moreis 8.7% to 15.9% and the value of Rtop required for achieving modulation(MOD) of 60% or more is 8.7% to 14.5%. Similarly, in the case where adata transfer rate of 140 Mbps taking the format efficiency to beapproximately 80% is achieved using the (1,7) RLL modulation code, sincethe required linear recording velocity is 21.0 m/sec, the value of Rtoprequired for achieving modulation (MOD) of 50% or more is 6.4% to 12.9%,the value of Rtop required for achieving modulation is (MOD) of 55% ormore is 6.4% to 11.7% and the value of Rtop required for achievingmodulation (MOD) of 60% or more is 6.4% to 10.7%.

Next, the physical structure of an optical recording medium that is apreferred embodiment of the present invention.

FIG. 3 is a drawing schematically showing the structure of an opticalrecording medium 10 that is a preferred embodiment of the presentinvention.

As shown in FIG. 3, an optical recording medium 10 includes a substrate11, a reflective layer 12 formed on the substrate 11, a seconddielectric layer 13 formed on the reflective layer 12, a recording layer14 formed on the second dielectric layer 13, a barrier layer 15 formedon the recording layer 14, a first dielectric layer 16 formed on thebarrier layer 15, a heat radiation layer 17 formed on the firstdielectric layer 16 and a light transmission layer 18 formed on the heatradiation layer 17. A hole 19 is provided in the center of the opticalrecording medium 10. When recording data onto an optical recordingmedium 10 with such a structure and reproducing data therefrom, a laserbeam is projected from the side of the light transmission layer 18.

The substrate 11 serves as a support for ensuring the mechanicalstrength required by the optical recording medium 10 and it ispreferable to set the thickness thereof to be 1.1 mm. While the materialfor forming the substrate 11 is not particularly limited, polycarbonatecan be used to from the substrate 11.

The reflective layer 12 serves to reflect the laser beam projected ontothe recording layer 14 via the light transmission layer 18 and emit itvia the light transmission layer 18 and the thickness thereof ispreferably set to be 10 to 300 nm. The material for forming thereflective layer 12 is not particularly limited but the reflective layer12 is preferably formed of an alloy containing Ag as a primarycomponent.

The second dielectric layer 13 mainly serves as a protective layer forthe recording layer 14 formed thereon and the thickness thereof ispreferably set to be 2 to 50 nm. While the material for forming thesecond dielectric layer 13 is not particularly limited, Al₂O₃, a mixtureof ZnS and SiO₂, CeO₂, Y₂O₃, AlN or the like can be used for forming thesecond dielectric layer 13.

The recording layer 14 is formed of a phase change material and data arerecorded therein utilizing the difference in reflection coefficientbetween the case where the recording layer 14 is in the crystal phaseand the case where it is in the amorphous phase. The reflectivity inthese states are set by adjusting the composition of the phase changematerial forming the recording layer 14 and, as explained above, theyare determined depending upon the target (attainable) linear recordingvelocity. The target (attainable) linear recording velocity is recordedin the optical recording medium 10 as recording condition settinginformation and when data are to be recorded, the recording conditionsetting information is read by a drive and data are recorded at a linearrecording velocity determined based on the thus read recording conditionsetting information. Here, the recording condition setting informationrefers to information used for identifying various conditions requiredfor recording data on the optical recording medium 10 concretelyincluding a linear recording velocity, the power of the laser beam, apulse train pattern described later in detail and the like. Therecording condition setting information may be recorded in the opticalrecording medium 10 in the form of wobbles or prepits or may be recordedin the optical recording medium 10 when recording data. Further, therecording condition setting information is not limited to specificallydesignated conditions required to record data but may also includerecording conditions designated by specifying one of several conditionsstored in advance in the information recording apparatus.

In order to change a region of the recording layer 14 in the crystalphase to the amorphous phase, the power of a laser beam projected fromthe side of the light transmission layer 18 is modulated in accordancewith a pulse waveform having an amplitude extending from a recordingpower (Pw) to a bottom power (Pb) so as to heat the recording layer 14to a temperature higher than the melting point thereof and then quicklycool it by setting the power of the laser beam to the bottom power (Pb).As a result, the phase of the region melted by the laser beam having therecording power (Pw) is changed to the amorphous phase, thereby forminga recording mark. On the other hand, in order to crystallize a region ofthe recording layer 14 in the amorphous phase, the recording layer 14 isheated to a temperature equal to or higher than the crystallizationtemperature thereof by setting the power of the laser beam projectedfrom the side of the light transmission layer 18 to an erasing power(Pe). The region of the recording layer 14 heated to the temperatureequal to or higher than the crystallization temperature thereof istherefore crystallized during the gradual cooling that follows.

Here, the recording power (Pw), the erasing power (Pe) and the bottompower (Pb) are set so that Pw is higher than Pe and Pe is equal to orhigher than Pb. Therefore, if the power of the laser beam is modulatedin this manner, it is possible to not only form a recording mark in anunrecorded region of the recording layer 14 but also directly overwrite(direct-overwrite) a recording mark already formed in a region of therecording layer with a different recording mark.

The material for forming the recording layer 14 is not particularlylimited but an SbTe eutectic crystal system material is preferably usedfor forming the recording layer 14. As the SbTe eutectic crystal systemmaterial, InSbTeGeTb is preferable. The thickness of the recording layer14 is preferably set to be 5 to 30 nm.

Similarly to the second dielectric layer 13, the first dielectric layer16 mainly serves as a protective layer for the recording layer 14 andthe thickness thereof is preferably set to be 10 to 300 nm. The materialfor forming the first dielectric layer 16 is not particularly limitedbut a mixture of ZnS and SiO₂ is preferably used for forming the firstdielectric layer 16.

The barrier layer 15 serves to prevent S (sulfur) contained in the firstdielectric layer 16 from reaching the recording layer 14 and thethickness thereof is preferably set to be 2 to 20 nm. While the materialfor forming the barrier layer 15 is not particularly limited, Al₂O₃,SiN, Y₂O₃ or the like can be used for forming the barrier layer 15.However, in the present invention, it is not absolutely necessary toprovide the barrier layer 15 and the barrier layer 15 may be omitted.

The heat radiation layer 17 serves to effectively radiate heat appliedto the recording layer 14 and to enlarge the power margin of the opticalrecording medium 10. Therefore, it is necessary for the heat radiationlayer 17 to have a thermal conductivity higher than at least the firstdielectric layer 16 and the heat radiation layer 17 is preferably formedof Al₂O₃, AlN or the like. The thickness thereof is preferably set to be10 to 200 nm and more preferably set to be 30 to 100 nm. However, in thepresent invention, it is not absolutely necessary to provide the heatradiation layer 17 and the heat radiation layer 17 may be omitted.

The light transmission layer 18 constitutes the incidence plane of thelaser beam and the thickness thereof is preferably set to be 10 to 300μm and more preferably set to be 50 to 150 μm. The material for formingthe light transmission layer 18 is not particularly limited but anultraviolet ray curable resin is preferably used for forming the lighttransmission layer 18.

Next, a drive capable of recording data in the optical recording medium10 according to this embodiment will be described.

FIG. 4 is a drawing schematically showing the major components of thedrive.

As shown in FIG. 4, the drive is equipped with a spindle motor 2 forrotating an optical recording medium 10, an optical head 3 for shining alaser beam onto the optical recording medium 10 and receiving the laserbeam reflected from the optical recording medium 10, a controller 4 forcontrolling the operation of the spindle motor 2 and the optical head 3,a laser driving circuit 5 that supplies a laser driving signal to theoptical head 3, and a lens driving circuit 6 that supplies a lensdriving signal to the optical head 3.

Moreover, as shown in FIG. 4, the controller 4 includes a focusing servocircuit 7, a tracking servo circuit 8, and a laser control circuit 9.When the focusing servo circuit 7 is activated, the focus is alignedwith the recording surface of the rotating optical recording medium 10,and when the tracking servo circuit 8 is activated, the spot of thelaser beam begins to automatically track the eccentric signal track ofthe optical recording medium 10. The focusing servo circuit 7 andtracking servo circuit 8 are provided with an auto gain control functionfor automatically adjusting the focusing gain and an auto gain controlfunction for automatically adjusting the tracking gain, respectively. Inaddition, the laser control circuit 9 is a circuit that generates thelaser driving signal supplied by the laser driving circuit 5 andgenerates a laser driving signal based on recording condition settinginformation recorded on the optical recording medium 10.

Note that the focusing servo circuit 7, tracking servo circuit 8 andlaser control circuit 9 need not be circuits incorporated in thecontroller 4 but can instead be components separate of the controller 4.Moreover, they need not be physical circuits but can instead beaccomplished by software programs executed in the controller 4.

When data are to recorded in the optical recording medium 10 accordingto this embodiment using the thus constituted drive, the recordingcondition setting information recorded on the optical recording medium10 is read and a linear recording velocity, the power of the laser beam,a pulse train pattern and the like are determined based on the recordingcondition setting information. The pulse train pattern to be used is notparticularly limited but considering that the present invention isparticularly effective in the case of recording data at a high linearrecording velocity, the (1,7)RLL modulation code is preferably used. Inthe (1,7)RLL modulation code, recording marks of lengths correspondingto 2T to 8T are formed in the recording layer 14.

FIG. 5 is a drawing illustrating the pulse train pattern in the case offorming a recording mark of a length corresponding to 2T.

As shown in FIG. 5, when forming a recording mark of a lengthcorresponding to 2T, the number of pulses in the laser beam is set to 1.Here, the number of pulses in the laser beam is defined by the number oftimes the power of the laser beam shined during recording is raised toPw. More specifically, taking the time t_(s) to be the timing at whichthe laser beam is positioned at the starting point of the recording markand the time t_(e) to be the timing at which the laser beam ispositioned at the ending point of the recording mark, during the periodfrom the time t_(s) to the time t_(e), the laser beam power is first setto Pw and then set to the power Pb. Here, the laser beam power beforethe time t_(s) is set to Pe and the power of the laser beam begins torise at the time t_(s). In addition, the laser beam power at the timet_(e) is set to Pe or Pb.

During the interval T_(pulse), the recording layer 14 of the opticalrecording medium 1 receives a large amount of energy and its temperatureexceeds the melting point, and during the interval T_(cl), the recordinglayer 14 of the optical recording medium 1 is rapidly cooled. Thereby, arecording mark of a length corresponding to 2T is formed in therecording layer 14 of the optical recording medium 1.

Similarly to the case of forming a recording mark of a lengthcorresponding to 2T, in the case of forming another recording mark of alength corresponding to one of 3T to 8T, the power of a laser beam isset to Pw, Pe or Pb and a recording mark of a predetermined length isformed using predetermined pulses.

As described above, according to this embodiment, since it is possibleto effectively suppress the degradation of jitter caused by recordingdata at a high linear recording velocity, in particular, at a linearrecording velocity equal to or higher than 10 m/sec, data can berecorded at higher data transfer rates.

The present invention is in no way limited to the aforementionedembodiment, but rather various modifications are possible within thescope of the invention as recited in the claims, and these are naturallyincluded within the scope of the invention.

For example, although the foregoing preferred embodiment was explainedwith regard to the optical recording medium 10 having the configurationshown in FIG. 3, the optical recording medium according to the presentinvention is not limited to the optical recording medium having such aconfiguration.

As described above, according to the present invention, since it ispossible to effectively suppress the degradation of jitter caused byrecording data at a high linear recording velocity, in particular, at alinear recording velocity equal to or higher than 10 m/sec, data can berecorded at higher data transfer rates.

WORKING EXAMPLE

First, optical recording media 10-1 to 10-4 like that shown in FIG. 3that had a substrate 11 consisting of polycarbonate and having athickness of approximately 1.1 mm, a reflective layer 12 consisting ofan alloy containing Ag as a primary component and having a thickness of100 nm, a second dielectric layer 13 consisting of a mixture of ZnS andSiO₂ (mole ratio of 50:50) and having a thickness of 3 nm, a recordinglayer 14 containing InSbTeGeTb and having a thickness of 14 nm, abarrier layer 15 consisting of Al₂O₃ and having a thickness of 5 nm, afirst dielectric layer 16 consisting of a mixture of ZnS and SiO₂ (moleratio of 80:20), a heat radiation layer 17 consisting of AlN and havinga thickness of 100 nm, and a light transmission layer 18 consisting ofan ultraviolet ray curable resin and having a thickness of 100 μm wereprepared. These optical recording media were different from each otheronly in the thickness of the first dielectric layer 16 and, as a result,the reflectivity (Rtop) of their recording layers in a crystal phasewere different from each other. The thickness of the first dielectriclayer 16 of the optical recording medium 10-1 was 30 nm, the thicknessof the first dielectric layer 16 of the optical recording medium 10-2was 40 nm, the thickness of the first dielectric layer 16 of the opticalrecording medium 10-3 was 42 nm and the thickness of the firstdielectric layer 16 of the optical recording medium 10-4 was 45 nm. Thephase change material forming each of the recording layers had acrystallization velocity optimized in the case of recording data at alinear recording velocity of 21.0 m/sec, namely, at a data transfer rateof 140 Mbps taking the format efficiency to be 80%.

Random signals consisting of recording marks of lengths corresponding to2T to 8T in the (1,7)RLL modulation code were recorded in each of theoptical recording media 10-1 to 10-4 under the conditions shown in Table1.

TABLE 1 Clock frequency 262.5 MHz Clock period (1T) 3.8 nsec Linearrecording velocity (CLV) 21.0 m/sec Modulation code (1,7) RLL Datatransfer rate 175 Mbps Format efficiency 80% Data transfer rate 140 Mbps(taking efficiency into account) Channel bit length 0.12 μm/bitNumerical aperture 0.85 Laser wavelength 405 nm Pw 7.0 to 8.0 mW Pe 2.2mW Pb 0.1 mW

Then, clock jitter of the random signals recorded in each of the opticalrecording media 10-1 to 10-4 was measured. When clock jitter wasmeasured, the fluctuation σ of a reproduced signal was measured using atime interval analyzer and the clock jitter was calculated as σ/Tw,where Tw was one clock period. The results of the measurement are shownin Table 2. In Table 2, the reflectivity (Rtop) of the recording layer14 in a crystal phase is shown for each of the optical recording media10-1 to 10-4.

TABLE 2 Jitter Rtop Optical Recording Medium 10-1 11.8% 12.5% OpticalRecording Medium 10-2 10.9% 11.0% Optical Recording Medium 10-3 10.4% 7.3% Optical Recording Medium 10-4 12.7% 17.0%

Here, the reflectivity (Rtop) of each of the optical recording media10-1 to 10-4 was measured as follows.

First, a plurality of media for measurement including only reflectivelayers 12 having different thicknesses were fabricated and a laser beamhaving the same wavelength as that of the laser beam to be used forrecording and reproducing data was projected onto a mirror region (aflat region formed with no groove or pit) of each of the media formeasurement and the reflectivity thereof was measured using aspectrophotometer, thereby obtaining the relationship between thethickness of the reflective layer 12 and the reflectivity for each ofthe media for measurement. Further, the laser beam which was to be usedfor recording and reproducing data and whose power was set to areproducing power was focused onto the mirror region of each of themedia for measurement using an evaluation apparatus and a sum signaloutput voltage value was measured for each of the media for measurement,thereby obtaining the relationship between the thickness of thereflective layer 12 and the sum signal output voltage value for each ofthe media for measurement.

Then, the relationship between the reflectivity and the sum signaloutput voltage value was derived for each of the media for measurementfrom the thus obtained relationship between the thickness of thereflective layer 12 and the reflectivity and relationship between thethickness of the reflective layer 12 and the sum signal output voltagevalue for each of the media for measurement.

Further, the laser beam whose power was set to the reproducing power wastracked along the groove on a region of the recording layer 14 in acrystal phase of each of the optical recording media 10-1 to 10-4 toobtain a sum signal output voltage value and the reflectivity of theregion of the recording layer 14 was calculated from the thus obtainedsum signal output voltage value and the relationship between thereflectivity and the sum signal output voltage value. The result wasdefined as Rtop. Rbtm can be calculated in a similar manner.

As shown in Table 2, jitter of the random signals became better as thereflectivity (Rtop) of the recording layer 14 in a crystal phase of theoptical recording medium was lower. Then, a graph showing therelationship between the reflectivity (Rtop) of the recording layer 14in a crystal phase and jitter was made using the values shown in Table2.

FIG. 6 is a graph showing the relationship between the reflectivity(Rtop) of the recording layer 14 in a crystal phase and jitter.

As shown in FIG. 6, it was found that there was a constant relationshipbetween the reflectivity (Rtop) of the recording layer 14 in a crystalphase and jitter and jitter decreased as the reflectivity (Rtop) of therecording layer 14 in a crystal phase was lower. Here, it was found thatthe reflectivity of the optical recording medium 10-1 satisfied theabove mentioned formula (3) and jitter was 11.8%, i.e., equal to orlower than the practical upper limit 12%, and it was found that thereflectivity of the optical recording medium 10-2 satisfied the abovementioned formula (5) and jitter was 10.9%, i.e., equal to or lower than11%. Further, it was found that the reflectivity of the opticalrecording medium 10-3 satisfied the above mentioned formula (6) andjitter was 10.4%, i.e., equal to or lower than 10.5%. To the contrary,it was found that the reflectivity of the optical recording medium 10-4did not satisfy the above mentioned formula (3) and jitter exceeded thepractical upper limit 12%.

1. An optical recording medium comprising a recording layer formed of a phase change material and capable of recording data therein at a linear recording velocity equal to or higher than 10 m/sec, wherein Rtop satisfies the condition that it is larger than {11−(V/5)} and smaller than {22−(2V/5)}, where Rtop (%) is a reflectivity of the crystal state of the phase change material forming the recording layer and V (m/sec) is a target linear recording velocity.
 2. An optical recording medium in accordance with claim 1, wherein Rtop and V satisfy the condition that Rtop is larger than {11−(V/5)} and smaller than {20−(4V/11)}.
 3. An optical recording medium in accordance with claim 1, wherein Rtop and V satisfy the condition that Rtop is larger than {11−(V/5)} and smaller than {18.3−(V/3)}.
 4. An optical recording medium comprising a recording layer formed of a phase change material and capable of recording data therein at a linear recording velocity equal to or higher than 10 m/sec and having recording condition setting information related to a linear recording velocity V (m/sec) that should be set when recording data, wherein Rtop satisfies the condition that it is larger than {11−(V/5)} and smaller than {22−(2V/5)}, where Rtop (%) is a reflectivity of the crystal state of the phase change material forming the recording layer.
 5. An optical recording medium in accordance with claim 4, wherein Rtop and V satisfy the condition that Rtop is larger than {11−(V/5)} and smaller than {20−(4V/11)}.
 6. An optical recording medium in accordance with claim 4, wherein Rtop and V satisfy the condition that Rtop is larger than {11−(V/5)} and smaller than {18.3−(V/3)}.
 7. A method of recording information to an optical recording medium comprising a recording layer formed of a phase change material whose reflectivity is Rtop (%) when it is in a crystal phase, which method records information under the condition that Rtop is larger than {11−(V/5)} and smaller than {22−(2V/5)}, where V (m/sec) is a linear recording velocity.
 8. A method of recording information to an optical recording medium in accordance with claim 7, wherein information is recorded under the condition that Rtop is larger than {11−(V/5)} and smaller than {20−(4V/11)}.
 9. A method of recording information to an optical recording medium in accordance with claim 7, wherein information is recorded under the condition that Rtop is larger than {11−(V/5)} and smaller than {18.3−(V/3)}. 